Force experienced by the surface is equal to change in momentum in one second. From Newton's second law of motion, the rate of change of momentum of a body is equal to the external force applied on the body and the change in momentum always takes place in the direction of the force. Momentum of ball before impact = mu Momentum after impact = - mu Change in momentum = mu - (- mu) = 2mu Change in momentum due to n balls in one second is = 2 mnu. Change of momentum = force experienced per second = 2 mnu.